In the builder there is an option to select multiple records and update them all at once.
I'd like to be able to do this on the app side. Similar to being able to do a bunch of inline editing actions in a batch.
My hope is that a feature like this would already have much of the programming already in place to implement this feature. 
Yes, this is really a UX shortcoming in this platform. One of the first things users are going to have problems with.
As soon as you check a “Inline editing” even if it is only one column that will be editable, the view should have a button “save changes”. So that the user can change all the fields and only submit the changes after all are edited.
As it is now, if you want to check or uncheck a box, you have to click 3 times, one to open the popup, then select the checkbox, and the third one to save the change. That is very user-unfriendly and not logic to do it this way. Knack has great potential, bit this is a very big disappointment.
In the next column, you can select rows for a batch update of Column C or D. Once you have selected some rows, click on the column head of Column C and enter your new value.
It is by no means perfect and could serve as a starting point for further ideas.